A conventional X-ray CT device has been known, which irradiates a subject to be examined with X-rays, detects X-rays transmitted through the subject or scattered by the subject with an X-ray detector, and provides a fluoroscopic image, tomogram, or three-dimensional image of the subject on the basis of the X-ray detection output (X-ray photon count).
Such X-ray CT devices are classified into a fan beam type and a cone beam type according to the beam shapes of X-rays. A general X-ray CT device uses a fan beam as an X-ray beam which is thin in the Z direction (a direction perpendicular to a tomographic surface). A cone beam CT device using a cone beam has recently been developed. This device uses an X-ray beam which also spreads in the Z direction. A CT device using a cone beam will be referred to as a cone beam CT device (CBCT device) hereinafter.
As this CBCT, a conventional CT (i.e., having only one row of detection elements) in a form equivalent to a so-called third-generation type or a scheme called an R/R type has recently been studied. The third-generation type (R/R type) CT is designed to perform scanning (acquisition of projection data) while making a pair of an X-ray source and a detector rotate around a subject to be examined.
FIG. 4 is a view showing an example of a CBCT device. The CBCT device shown in FIG. 4 belongs to third-generation type CT devices, and makes both an X-ray source (X-ray tube 401) and an X-ray detector 402 pivot around a subject to be examined. They make one rotation to complete scanning a region of interest.
In a general X-ray CT device, detection elements are arranged in one line in a channel (CH) direction to sample in this direction. Each element is identified by a channel number. In contrast to this, in a CBCT device, as shown in FIG. 4, detection elements are also arranged in the Z direction (row direction). That is, the detector of the CBCT device has detection elements two-dimensionally arranged in the form of an orthogonal lattice.
According to such a CBCT device, detection elements are arranged in the two directions, i.e., the Z direction (row direction) and CH direction, in the form of a lattice to form a detector, and radiation is applied in the form of a cone by making it have a thickness in the Z direction as well, thereby obtaining projection data corresponding to a plurality of columns at once.
X-ray imaging diagnosis generally uses both some radiography and X-ray CT. Radiography is used for initial diagnosis, whereas CT is used for detailed diagnosis. It, however, does not mean that CT images make radiography unnecessary because general radiography makes it possible to comprehensively grasp the condition of a patient.
A positioning image (called a scanogram, scout image, or the like) is sometimes taken by using a CT sensor before CT imaging. This image can be regarded as a projection image such as an image obtained by radiography. A scanogram is described in, for example, Japanese Patent Laid-Open No. 2001-218767. In general, a CT sensor is about 1 mm square, and hence lacks in resolution. The resolution of a scanogram is therefore not enough to be used for diagnosis based on radiography. In order to obtain both a CT image and a radiography, an X-ray CT device and X-ray imaging device need to be separately prepared. That is, a plurality of devices (facilities) must be purchased.
The present invention has been made in consideration of the above problems, and is configured to acquire radiographys during acquisition of CBCT data in a CBCT device using an FPD (Flat Panel Detector) in consideration of the fact that when the FPD is sufficiently large in size, CT projection data itself has an area corresponding to radiography. In this case, problems are posed in terms of resolution and radiation dose. In general, CT data are acquired at high speed (1,000 frames per sec), but low in resolution. In addition, the S/N ratio of such data is lower than that of data obtained by radiography. This may make it necessary to change the imaging form during CT scanning.
As will be described below, arrangements designed to change the imaging form during scanning are described in Japanese Patent Laid-Open Nos. 08-299321, 05-212022, and 06-054838. None of these arrangements is aimed at obtaining a radiography for diagnosis during CT scanning, and there is no indication of an arrangement which can realize it.
Japanese Patent Laid-Open No. 08-299321 has an object to solve the following problem. When imaging is to be continuously done by helically scanning the first and second imaging regions while the rotational speeds (scan speeds) of an X-ray source and detector are changed, the positional information of a top (table on which a subject to be examined is placed) which is calculated by a data acquisition electronics (DAS) after the start of change in rotational speed deviates from the actual positional information of the top. This makes it impossible to accurately obtain a tomogram of a desired region.
According to Japanese Patent Laid-Open No. 08-299321, the rotational speeds of the X-ray and X-ray detector can be changed, and the translational speed of the top is controlled in accordance with the rotational speeds of the X-ray and X-ray detector. When at least the rotational speeds of the X-ray and X-ray detector change, the current rotational speeds of the X-ray and X-ray detector are obtained on the basis of the positional information of the X-ray and X-ray detector which is detected by a rotational position detection means, and the translational speed of the top is controlled on the basis of this positional information.
As described above, Japanese Patent Laid-Open No. 08-299321 discloses an arrangement which acquires data while changing the scan speed of the CT device. However, this arrangement is not designed to obtain a radiography for diagnosis during CT scanning, and there is no description about of it.
Japanese Patent Laid-Open No. 05-212022, describes a data recording method wherein the scan/rotational speed or bed speed is changed. In an embodiment, there is the description “Scanning is continuously completed by changing imaging conditions in accordance with the region to be imaged. For example, for a chest portion, the movement amount of the bed is set to be relatively small to increase the resolution, whereas for an abdominal portion, the movement amount of the bed is set to be relatively large.” This technique is aimed at increasing the slice resolution in the body axis direction by adjusting the movement amount of the bed, but does not imply a change in data resolution when the sensor itself, which performs data acquisition, is taken into consideration. That is, with the technique disclosed in Japanese Patent Laid-Open No. 05-212022, a radiography for diagnosis cannot be obtained either during CT scanning, and there is no description of it.
Japanese Patent Laid-Open No. 06-054838 discloses a CT device in which when different speed ranges are input and set depending on the region to be screened and the region to be closely examined, command signals corresponding to the settings are supplied from the rotational speed commanding device of a motor to a servo amplifier. The servo amplifier then rotates/drives the motor in accordance with the above settings to cause a bed driving mechanism to move the bed at different moving speeds for the region to be screened and the region to be closely examined. A bed position detector feeds back the position data of the subject on the bed to the rotational speed commanding device of the motor to make a correction such that command signals are supplied from the rotational speed commanding device of the motor to the servo amplifier in accordance with the settings made by a speed setting device.
For example, switching control of the moving speed of the bed is performed to scan the cerebral basal region with a slice width of about 1 or 2 mm, and the cerebral region with a slice width of about 5 or 10 mm, thereby shortening the imaging time without degrading the image quality of an image of the cerebral basal region. This reference also discloses a technique of performing switching control of the rotational speed of the X-ray (gantry) in the data acquisition unit, i.e., the changing speed in the projection direction instead of shortening the imaging time and controlling artifacts by switching control of the moving speed of the bed.
According to Japanese Patent Laid-Open No. 06-054838, the resolution of slice data for reconstruction is changed depending on the region to be imaged on the basis of the relationship between the table speed and the scan speed. Obviously, however, this reference discloses no technical idea of obtaining a radiography for diagnosis during CT scanning.